<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
  <script>
    /*let arr= new Array(3);
    document.write(arr.length);
    let arr1 = new Array(3,2,1);
    alert(arr1);*/
    let arr = [2,1,5,6];
    arr[4]=9;
    /*alert(arr.join(":"));*/
    //alert(arr.sort())
    alert(arr.reverse())
  </script>
</head>
<body>

</body>
</html>